I found 5 different stores on Maui, 3 were in Lahaina and 2 were in or near Kahului. This does not count the a number of gift shops that have a laminate or three to sell, not to mention the tourist ones for $20.
In Lahaina, you have Lahaina Music right on Hwy 30 and Lahainaluna Rd. They have a good selection and offer a free lesson over at the Lahaina Cannery Mall on Tuesday nights, they'll even provide the Uke. The next Lahaina location is called Kula Gold and is located in the Pioneer Inn on Front St. It's a small store, Mike is the owner, and he has a good selection. Mike is good to deal with and will take care of you if he has something you are interested in.
The last shop in Lahaina is a small shop off the beaten path. The sell off brands of hand made Ukes but I did not recognize the brands like D&D. They were a little pushy for my taste but the are near Lahainaluna and Front St, its actually behind the building...
In Kahului you have Bounty Music, right off the Hana Highway. They have a large selection of Ukulele's and were very helpful and informative. The other store in Kahului area is actually in Wailuku and it's Mele Ukulele, it's a small store with a ton of Ukulele's. I bought my last one there and they were very helpful and friendly. Cheryl and Eva will take good care of you and will help you find what you are looking for. While they are located there in Maui, the Ukulele's are actually constructed in the Phillipines, but they are finished and setup there in Maui. I had a great experiecne and woudl recommend them to anyone.
Enjoy your trip and don't forget to stop at Momma's Fish House for a great meal!